Adapted by Helen Edmundson from Jamila Gavin's novel, Coram Boy is messy, helter-skelter, awhirl with melodramatic plots and confusing leaps in time, but it produces one of the rarest sounds in today's theatre: sniffing into hankies.

Strike a chord of The Messiah as Christmas approaches and you're likely to tap into a tear duct, but the use of Handel's music in Melly Still's production is exceptional. It's not only that Adrian Sutton's arrangements are exquisite and that they are delivered by choir and soloists with gravity and sweetness. The music fuels the plot. This is a story of 18th-century boys who leave their families to become musicians and meet Handel himself; it's also a tale of abandoned, murdered and disinherited children. In the course of the evening, 'Unto us a child is born' begins to take on new shades of desolation.

Staged beneath a huge organ amid a forest of tall pipes, the action is a crash course in the 18th century: slavery, foundlings, low-life, public buttock-fondling, buckled shoes and stockinged legs, public hangings, huge sailing ships and dainty trills on the virginals. Melly Still, who designs together with Ti Green, draws constantly on her work in movement theatre: nothing is plonkingly literal. A ship is conjured up by looping ropes across an empty stage; a wall of water is created by a huge, transparent sheet, against which a drowning man throws himself towards the audience; there are puppet babies and puppet baby skeletons.

It takes a few minutes to adjust to adults of disconcertingly different sizes bouncing around in infant pinnies and pantaloons. But in a play where, as so often, the best parts for girls are as boys, there are some extraordinarily natural lad performances from Anna Madeley, the serious chorister hero, Abby Ford as her mischievous, doomed pal, and Akiya Henry as the foundling who's kept as a piccaninny pet, dressed up to look like a toffee and told to keep grinning. Stalking the lot of them as the child-catcher villain, Paul Ritter is perfectly smarmy and sinister.

There's a real exhilaration in seeing the National push forward from His Dark Materials, continuing to lead the subsidised theatre away from the primness of Victorian children's books and, far more important, from the punitive and sanctimonious works of CS Lewis. Coram Boy will, no doubt, be trimmed and smoothed and clarified as it continues to be produced, but, thanks to Handel, it already has not just heart but wings.

There's a strange thing that Juliet Stevenson does with her face, something that seems almost involuntary, as if it's been visited on her. She can be quite far away from an audience, and absolutely still on the stage and yet persuade you that you're seeing her in close-up. It's not to do with being more vehement or animated than other actresses; it's not that she has more expressions (she has none of the rippling, multi-inflected quality of Vanessa Redgrave); yet she looks if she's got internal lighting.

She needs and uses this in Alice Trilogy. Tom Murphy, the veteran Galway playwright who, in the Seventies helped the Catholic church to reword the Mass, has written a triptych of scenes which has the effect of a fractured monologue, a soliloquy which only occasionally bumps into the outside world. In the 1980s, Stevenson is a mother in her twenties: fed up, not being able to say exactly why and hitting the bottle. Fifteen years later, she's telling a former lover how unfulfilled she is. In her fifth decade, she is bereaved and unable to talk about her grief to her husband.

All this has both the self-absorption and the tedium of true depression: you see a woman trapped behind her own face like Alice behind the looking-glass. Even when there are other strong actors on stage (Derebhle Crotty is remarkable as a waitress whose misery sits on her like a hump), they are simply obstacles in the flow of her bewildered sadness.

Directed with exemplary intensity by Ian Rickson, Stevenson suggests a weird dislocation between her face and her mind; she can perch a smile on the top of her mouth and suggest that several different things are happening underneath it. But after two hours, it's hard not to side with her much-abused husband, deemed by her to be dull since he's not only a banker but a keeper of budgerigars.

It's one of the most famous and striking of theatrical ironies. In 1673, Moliere was taken ill during the fourth performance of his Le Malade imaginaire, and died shortly afterwards. He had been taking the title role of the hypochondriac.

Writer Richard Bean wittily adds this event as an epilogue to his rousing, clever - and tirelessly scatological - new version of the play. It's a poo-packed evening: Giles Cadle has designed an elegant salon decorated with rows of specimen ja rs displaying the patient's turds; Bean has a strong line in 'with friends like that, who needs enemas?' jokes; a blazing Henry Goodman guards his commode as intently as if it contained a potty of gold.

Lindsay Posner's production is too full of japes to bring off the shift in the play from knockabout to a finely honed satire on superstition and irrationality, but Bean expresses the change with considerable eloquence, and a strong cast bounce the evening along.

Carey Mulligan is fresh as a daisy and never saccharine as the young heroine; Ronni Ancona, the rapacious stepmother, coos over her hated husband and shifts uneasily on her hams when he mentions childbearing; John Marquez, as the hopelessly rigid suitor, moves as if he has been mummified and offers the object of his affections a date watching him dissect a corpse.

Bean thinks of the play as one of the first modern dramas, driven by the playwright's own obsession. He makes his case wonderfully well.